1. Field of the Invention
This invention relates to fail-safe circuit for a constant speed drive apparatus for inhibiting a constant speed drive control when abnormal control or out of order of the constant speed drive apparatus is detected.
2. Description of the Prior Art
A constant speed drive apparatus is a mechanism which controls the opening degree of a throttle valve of an internal combustion engine to maintain a preset vehicle speed. The apparatus controls the throttle opening degree in an open-throttle direction when actual vehicle speed is lower than the set speed, thereby to increase the vehicle speed, and when actual vehicle speed is higher than the set speed, the apparatus controls the throttle opening degree in a closed throttle direction to lower the vehicle speed.
However, in the case where any abnormal condition of control exists such that the throttle valve is controlled in the open throttle direction despite the fact that actual vehicle speed is equivalent to or higher than the set speed or that constant speed drive control has been released, the vehicle speed is increased contrary to the driver's intention. Such condition is extremely dnagerous.
In order to avoid such danger, the constant speed drive apparatus are conventionally equipped with fail-safe circuit which monitor the signal level of a signal line for controlling the throttle valve.
FIG. 1 is a block diagram of a conventional constant speed drive apparatus equipped with fail-safe circuit. An actuator 71 for actuating a throttle valve of the internal combustion engine mounted in an automobile is constituted by a release valve 72, and a control valve 73. A processing circuit 74 implemented by a microcomputer or the like energizes the release valve 72 to communicate the atmosphere, and controls a duty ratio of the control valve 73, thereby utilizing a negative pressure of an intake manifold to control the opening degree of the throttle valve, the automobile being thus controlled for constant speed driving.
When the constant speed drive apparatus is energized by a power supply, a fail-safe circuit 75 outputs a high level signal through a signal line l13 to set switches SW11-SW13 to turning on. A low level output from an output terminal OUT14 of the processing circuit 74 is supplied to a base of a transistor TR16 through a gage G12 which is a non-inverting buffer, to turn on the transistor TR16. Accordingly an exciting current flows through the signal line l11 to energize the release valve 72. Signals for change to high and low levels for controlling the duty ratio are supplied from an output terminal OUT13 of the processing circuit 74 to a transistor TR15 through a non-inverting buffer gage G11. Whereby an energizing current flows in the control valve 73 through a signal line l12, the control valve 73 being thus controlled by the duty ratio.
The constant speed drive apparatus goes into a most dangerous condition when the control valve 73 is continuously energized with the result that the vehicle speed only tends to abruptly increase. In order to avoid such condition, a typical prior art apparatus for driving at the constant speed is provided with a fail-safe circuit 75 for constantly monitoring the signal level on the signal line l12 which actuates the control valve 73. Whereby when the fail-safe circuit 75 detects any abnormality with the signal level, the electric power form the power supply is cut off to the actuator 71 so as to prevent any abrupt increase in vehicle speed.
In FIG. 1, when the fail-safe circuit 75 receives the signal from the signal line l12 and a high level signal is supplied to the control valve 73 of the actuator 71 over a predetermined time, the fail-safe circuit 75 determines that an abnormal condition exists and accordingly outputs a low level signal to the signal line l13 to turn off the switches SW11-SW13 on one hand while, on the other hand, the fail-safe circuit 75 supplies a signal indicating the abnormal situation to the processing circuit 74 through a signal line l14. The release valve 72 is deenergized by the switches SW11-SW13 being turned off and, as a result, the release valve 72 is opened the atmosphere and the throttle valve is angularly displaced in the close throttle direction, the vehicle speed being then lowered. It may be noted in this apparatus that not all the switches SW11-SW13 are needed; there should be at least one of them.
With the above described prior art arrangement, in the case where the fail-safe circuit 75 itself is out of order abnormal signals leading out to the control valve 73 can not be detected. For example, when an emitter-collector short circuit should occurs in the transistor TR15, high level signals are outputted continuously to the signal line l12 and the control valve 73 is energized accordingly, with the result that the vehicle speed is abruptly increased. Such trouble is very dangerous and the condition would become further serious, and is coupled with any trouble in the constant speed drive apparatus. Another difficulty is that the constant speed drive apparatus has no function to detect whether the fail-safe circuit is out of order or not.